1. Field of the Invention
The present invention relates to a switching power supply circuit, and more particularly, to a switching power supply circuit having an overcurrent detecting function and an output short circuit detecting function.
Moreover, the present invention relates to a switching power supply circuit capable of protection against degradation and breakage, due to an overcurrent or a short circuit current, of a switching element and an inductive load circuit and the like fed by the switching element.
2. Description of the Prior Art
In recent years, the need for protection against degradation and breakage of parts under abnormal conditions has been increasing with decrease in voltage and increase in current in semiconductor integrated circuits.
Referring now to FIG. 3, a conventional switching power supply circuit will be described.
The conventional switching power supply circuit comprises as shown in FIG. 3: an error amplifier 18 detecting the voltage at an output terminal Vout; a switching element 19 whose on/off is controlled by the output of the error amplifier 18; a constant current source 20; a capacitor 21 charged by the constant current source 20; a comparator 22 detecting the charging voltage of the capacitor 21; a latch circuit 23 receiving as an input the output signal of the comparator 22; an activating circuit 26 resetting the latch circuit 23; a control signal generating circuit 24; an AND circuit 25 serving as an output drive circuit; a MOS transistor M2 serving as a switching element; a choke coil 29; a Schottky diode 30; resistors 31 and 32 for output voltage detection; and a capacitor 33 for output smoothing. Reference numeral 27 represents a direct-current power supply terminal. Reference numeral 28 represents a direct-current power source. The choke coil 29, the Schottky diode 30, the output voltage detecting resistors 31 and 32 and the output smoothing capacitor 33 constitute an inductive load circuit.
The operation of the switching power supply circuit structured as described above will be described.
Under normal operation conditions, the MOS transistor M2 is switched on and off by a pulse width modulation signal generated by the control signal generating circuit 24 in accordance with the voltage at the output terminal Vout. When the MOS transistor M2 is on, electric power, or energy is supplied from the direct-current power source 28 to the choke coil 29, the capacitor 33 and the output load. At this time, energy is stored in the choke coil 29. When the MOS transistor M2 is switched off by the pulse width modulation signal, a counter electromotive force is caused at the choke coil 29, so that a regenerative current flows through the Schottky diode 30. By smoothing by the capacitor 33 the voltage caused at the choke coil 29 at this time, a direct-current voltage is obtained. The direct-current voltage is output to the output terminal Vout.
The control signal generating circuit 24 generally includes a triangular wave generator (not shown) and an error comparator (not shown). The error comparator monitors the normally output voltage, or the voltage at the output terminal Vout and compares the voltage with the output signal of the triangular wave generator, thereby generating the pulse width modulation signal whose pulse width varies according to the voltage at the output terminal Vout. However, since this is not the essence of the invention, description thereof is omitted.
When the inductive load circuit is overloaded or the output terminal Vout is short-circuited under abnormal conditions, the output voltage detecting error amplifier 18 detects that the potential at the output terminal Vout is decreased, and outputs a high level, thereby switching off the switching element 19. At the same time, the charging of the capacitor 21 is started by the constant current source 20. This operation is maintained during a period for which the voltage at the output terminal Vout is lower than a predetermined voltage and the error amplifier 18 is generating an inversion signal. During this period, the charging of the capacitor 21 is continued, and when a predetermined time determined by the current value of the constant current source 20 and the capacitance value of the capacitor 21 elapses and the charging voltage of the capacitor 21 exceeds a reference voltage Vx of the comparator 22, the comparator 22 sets the latch circuit 23 and switches off the MOS transistor M2. By doing this, the MOS transistor M2 and parts of the inductive load circuit and the like are protected from breakage or degradation.
In the above-described conventional structure, when the output of the switching power supply circuit is overloaded or short-circuited to decrease the voltage at the output terminal Vout, a protection function of causing the MOS transistor M2 to be off for a predetermined period of time works. However, in the overloaded or the short-circuited condition, since no current limitation is imposed, an unlimited overcurrent continuously flows through the MOS transistor M2 and the inductive load circuit, or the choke coil 29, the Schottky diode 30, the resistors 31 and 32 and the capacitor 33 from the start of the timer to the setting of the latch circuit 23. As a result, there is a possibility that the MOS transistor M2 or the inductive load circuit is degraded or broken down.
An object of the present invention is to provide a switching power supply circuit capable of protecting the switching element and the parts constituting the inductive load circuit with higher reliability.
A switching power supply circuit of the present invention comprises: a direct-current power source; an inductive load circuit supplied with electric power from the direct-current power source; a switching element interrupting the electric power supplied from the direct-current power source to the inductive load circuit; a control signal generating circuit generating a control signal periodically bringing the switching element into conduction; an overload detecting circuit generating an overload detection signal when a load current flowing through the switching element exceeds a predetermined current value; a first latch circuit being set by the overload detection signal and being reset in response to a leading edge of the control signal; a timer circuit performing a clocking operation during a period for which the first latch circuit is set, and generating a time-up signal when a predetermined clocking period elapses; a second latch circuit being set in response to the time-up signal of the timer circuit; and an output drive circuit receiving as inputs the control signal and output signals of the first and the second latch circuits, bringing the switching element into conduction in response to the control signal under normal conditions, and shutting off the switching element irrespective of the control signal when at least one of the first and the second latch circuits is set.
According to this structure, the overload detection signal is generated when the load current flowing through the switching element exceeds the predetermined current value, and at this time, the switching element is shut off irrespective of the presence or absence of the control signal, so that the on period of the switching element is reduced. Consequently, the maximum current value can be limited to not more than a predetermined value. Further, at the time of the occurrence of an abnormal current when the overloaded condition continues, the control of the switching element can be stopped so that the switching element is held in the shut-off condition. Consequently, the protection against degradation and breakage of the switching element and parts of the inductive load circuit can be made more reliable.
In the switching power supply circuit having the above-descried structure, for example, it is preferable that the overload detecting circuit compares an on voltage of the switching element with a predetermined first reference voltage in synchronism with switching on of the switching element to thereby detect that the load current flowing through the switching element exceeds the predetermined current value.
According to this structure, since the overcurrent or the output short circuit current is detected by using the on resistance of the switching element, no special current detecting resistor is necessary. Consequently, only a small number of parts are required, and since there is no power loss at the current detecting resistor, the power source use efficiency never decreases.
In the switching power supply circuit having the above-described structure, for example, the timer circuit comprises: a capacitor; charging means for passing a charging current through the capacitor during the period for which the first latch circuit is set; discharging means for passing a discharging current through the capacitor during a period for which the first latch circuit is reset; and a comparator comparing a charging voltage of the capacitor with a predetermined second reference voltage and outputting the time-up signal when the charging voltage of the capacitor exceeds the second reference voltage.
According to this structure, when it is detected that the load current of the switching element becomes an overcurrent, that is, that the switching element is in the overloaded condition, the charging of the capacitor is started, and the clocking operation is performed while the succeeding charging and discharging of the capacitor are repeated, so that the time-up period that lasts until the charging voltage of the capacitor reaches the second reference voltage is longer than that of the conventional example. Consequently, the capacitance value of the capacitor can be made lower that that of the conventional structure. That is, the time-up period can be made long although the capacitance value of the capacitor is low. Consequently, the capacitor can be formed so as to be small, and this enables the timer circuit including the capacitor to be integrated into a semiconductor integrated circuit.
Moreover, it is preferable that in the timer circuit comprising the capacitor, the charging means, the discharging means and the comparator, a level of the second reference voltage is set so that a period from start of a charging and a discharging operation to generation of the time-up signal, that is, a time-up period is not less than twice a period of the control signal. Further, it is more preferable that the period is three to five times a period of the control signal
According to this structure, even when the capacitor is charged as a result of a one-shot noise intruding from a peripheral circuit or a peripheral apparatus to cause the first latch circuit to malfunction, the discharging means functions to discharge the capacitor so that the charging voltage decreases. Consequently, even when a one-shot noise intrudes from a peripheral circuit or a peripheral apparatus, it is avoided that the operation of the switching element is stopped by a malfunction due to the noise, so that a highly reliable overcurrent detecting operation can be performed.
Moreover, it is preferable that in a switching power supply circuit comprising the timer circuit comprising the capacitor, the charging means, the discharging means and the comparator, the following are further provided: an activating circuit a period of generation of which output signal is set to a period longer than a period of the control signal; and short-circuiting means for short-circuiting across the capacitor in response to the setting of the second latch circuit, and the second latch circuit is periodically reset by an activation signal of the activating circuit.
According to this structure, when an operator in charge of operating an electronic apparatus provided with the switching power supply circuit of the present invention makes a mistake in the operation of the apparatus to short-circuit the inductive load circuit, not only the overloaded condition is detected and the switching element is shut off to thereby protect the circuit but also the shut-off condition can be periodically canceled. Consequently, by the operator eliminating the short-circuited condition, the overcurrent detection operation for causing the protection operation to function is resumed and the primary function of the switching power supply circuit can be delivered.